Determine the efficacy of low-dose ondansetron in the treatment of obsessive-compulsive disorder. After the nine week trial of treatment w/ondansetron, assessment of the OCD patients completing this trial as specified will show a significant change* in the Yale Brown Obsessive Compulsive Scale (YBOCS) score. Scores of a patient self rating scale (PSRS), and the YBOCS Clinical Global Impression (CGI) rating will show significant changes as secondary measures. Over the nine week period of treatment w/ondansetron, at least 1/3 of OCD patients entering the study will show a clinically significant improvement (25% reduction) of their OCD symptoms which will manifest in their total YBOCS scores.